1. Field of the Invention
The present invention relates to a photomask and, more particularly, to a photomask for use in the production of liquid crystal displays (LCDs) which are used as monitor displays in notebook computers, desk-top computers, car navigation systems, and in wall-hung TVs. The present invention also relates to a method for producing a TFT substrate and a display device using a photomask.
2. Description of the Related Art
In recent years, flat panel displays (FPDs) such as liquid crystal displays (LCDs) are widely used as display devices in personal computers or thin TVs. Efforts have been made in the art to increase the area of an FPD substrate in order to increase the area of these display devices. Accordingly, the area of a photomask used in the production of such an FPD substrate has also been increased.
In order to produce such a photomask having a large area, efforts have been made in the art to provide a mask drawer (drawing device) capable of high speed and large-area drawing operations. For example, Japanese Patent No. 2696364 and Japanese Laid-Open Publication No. 5-326356 disclose a type of a mask drawer, i.e., a raster scan type pattern drawing device. A conventional mask drawer capable of large-area drawing operations will now be described.
FIG. 1 illustrates a conventional mask drawer 100. The mask drawer 100 illustrated in FIG. 1 includes a pattern drawing laser light source 101, an acousto-optical modulator (AO modulator) 102 for changing the diffraction angle of a beam from the light source according to an input voltage, a zoom lens 103, an optical head 104, an optical head support guide 105 for allowing the optical head 104 to be moved along the xe2x80x9cXxe2x80x9d direction (as indicated in FIG. 1), an optical stage 107 on which a material sheet 106 is placed, and a mounting table 108 on which the optical stage 107 is mounted. The optical head 104 accommodates a polygon mirror which allows for a wide area exposure by repeatedly scanning the material sheet 106 with a beam, an objective lens for controlling a beam L1 to be incident upon the material sheet 106, etc. The optical stage 107 is mounted on the mounting table 108 so that the optical stage 107 is movable along the xe2x80x9cYxe2x80x9d direction (as indicated in FIG. 1). In the mask drawer 100 illustrated in FIG. 1, the optical head 104 and the optical stage 107 are movable along the X direction and the Y direction, respectively. Alternatively, the optical head 104 may be fixed, with the optical stage 107 being movable along the X and Y direction. In any case, the optical head 104 can be positioned at an intended position above the material sheet 106.
Typically, the optical head 104 and the optical stage 107 are provided with laser length measuring devices 110 and 111, respectively. The length measuring devices 110 and 111 use a light beam L2 from a laser light source 109. Thus, it is possible to precisely position the optical head 104.
Next, a mask drawing method using the conventional mask drawer 100 will be briefly described.
First, the optical head 104 is positioned at an intended position above the material sheet 106 (e.g., the lower left corner of the material sheet 106).
The optical head 104 is then operated to scan the material sheet 106 with the beam L1, deflected by the AO modulator 102 by a predetermined interval along the Y direction. Moreover, the optical head 104 (or in some cases the optical stage 107) is moved along the X direction in synchronism with the rotation of the polygon mirror provided in the optical head 104. Thus, a strip-shaped portion 112is drawn with a constant width D1 along the Y direction. The width D1 corresponds to the predetermined interval by which the optical head 104 is moved along the Y direction.
Then, the optical stage 107 is stepped by an appropriately adjusted drawing pitch of the mask drawer 100 which corresponds to the width D1 of the drawn strip-shaped portion 112 along the Y direction. Thus, the optical head 104 is positioned at a position above the material sheet 106 adjacent to the previously drawn strip-shaped portion 112. Thereafter, by a process as described above, another strip-shaped portion having a predetermined width is drawn adjacent to the previously drawn strip-shaped portion 112. The above-described process is repeated in a raster scan manner so that a pattern is drawn across the entire surface of the material sheet 106.
Typically, the width D1 of the strip-shaped portion 112 may be several tens to several thousands of micrometers. In the LCD field, for example, the range of the width D1 suitable for practical use is several hundreds of micrometers. Minimizing the positional and dimensional errors in the laser-drawn areas (the strip-shaped portions) which are adjacent to one another is important for drawing an intended pattern across the entire surface of the material sheet 106. Various methods have been developed in the art to minimize the positional and dimensional errors in the laser-drawn areas. Two of such conventional methods will now be described.
The first conventional method is a multiple exposure method in which a sufficient overlap is provided between two adjacent exposed areas (laser-drawn areas). The first conventional method is performed by first exposing a first area with an amount of exposure which is equal to, for example, xc2xc of the exposure sensitivity of a resist material used, then exposing a second area having the same size as the first area and displaced from the first area by xc2xc of the area with the same amount of exposure (i.e., xc2xc of the exposure sensitivity of the resist material), and so forth. In this method, a sufficient amount of overlap is provided for one exposed area (as a result, each area is exposed four times). In this way, although the positional and dimensional shift may possibly occur in each exposure step with the amount of xc2xc of the exposure sensitivity of the resist material, for the whole process such a shift is compensated for. As a result, such a shift can be reduced. Therefore, it is possible to perform a drawing process with a very high positional and dimensional precision.
As described above, according to the first conventional method, a large amount of overlap is provided between two successive areas to be exposed, thereby considerably reducing the throughput of the mask production. Thus, this method has not been practical for large-area mask drawing process in terms of the cost. In fact, the application of this method has been limited to a process of drawing a relatively small-area pattern such as a mask used in a stepper (reticle).
The second conventional method is a drawing method for large-area masks in which the amount of overlap exposure provided for a transitional portion is minimized, thereby improving the production throughput.
The term xe2x80x9ctransitional portionxe2x80x9d as used herein refers to an overlap, a gap or a boundary between two adjacent drawn areas (exposed areas) on the material sheet to be exposed.
The second conventional method will now be described.
FIG. 2 illustrates a portion of a photomask exposed according to the second conventional method around a transitional portion. Referring to FIG. 2, a first drawn area (first exposed area) 201 and an adjacent second drawn area (second exposed area) 202 overlap each other over a transitional portion 210.
According to the second conventional method, the amount of exposure for each area to be exposed is controlled in a graded manner (in a xe2x80x9cstepped triangular patternxe2x80x9d) with four different exposure levels (25%, 50%, 75% and 100%). In each transitional portion of two adjacent areas, a synthetic pattern obtained by synthesizing together the respective drawing patters is drawn. Thus, the transitional portion 210 is exposed to an amount of light which is determined according to a synthetic exposure profile 205 or 206 which is obtained by synthesizing together a first drawn area exposure profile 203 and a second drawn area exposure profile 204. The synthetic exposure profile 205 or 206 is ideally a straight line. However, when actual transition positions 208 and 209 are shifted from an intended transition position 207 by a minute positional shift amount (xc2x1xcex94d), the synthetic exposure profile 205 or 206 exhibits a rectangular protrusion corresponding to the minute positional shift amount (xc2x1xcex94d).
In the coordinate system illustrated in FIG. 2, when the minute positional shift amount in the second exposed area 202 has a positive value (xcex94d) (e.g., when the actual transition position is located at a position indicated by a broken line 208), an overexposure of 125% of the optimal exposure occurs locally as indicated by the synthetic exposure profile 205. Conversely, when the minute positional shift amount in the second exposed area 202 has a negative value (xe2x88x92xcex94d) (e.g., when the actual transition position is located at a position indicated by a broken line 209), an underexposure of 75% of the optimal exposure occurs locally as indicated by the synthetic exposure profile 206.
Thus, according to the second conventional method, it is possible to reduce the positional and dimensional error between two adjacent drawn areas while minimizing the amount of overlap in the transitional portion. This is because each transitional portion between two adjacent drawn areas is exposed to an amount of light which is obtained by averaging the respective amounts of exposure for the two areas (i.e., according to the synthetic exposure profile). Moreover, according to the second conventional method, the height or depth (amplitude) of the rectangular protrusion occurring in the synthetic exposure profile corresponding to the minute positional shift amount xc2x1xcex94d is decreased.
However, there may still occur a shift in the amount of exposure corresponding to the above-described minute positional shift in the transitional portion or the error in the beam irradiation power (e.g., an area which is intended to be exposed with a 100% exposure may be exposed with a 98% exposure). Due to this problem, the second conventional method currently has a pattern drawing precision which is inevitably lower than that of the first conventional method by an order of magnitude. Moreover, FIG. 2 illustrates an example in which the exposure profile has an ideal stepped triangular pattern. In an actual process, however, there may be some variations in the deflected beam intensity which is controlled by the input voltage to the AO modulator. This may also disturb the drawn pattern. Moreover, it may be necessary to provide each transitional portion in the form of a strip-shaped area having a width of several tens of micrometers in order to avoid a substantial change in the positional and dimensional precision between two adjacent exposed areas.
Furthermore, the present inventors have found that the following problems may further occur when a mask used in an FPD substrate is drawn by using the second conventional method.
Where an FPD substrate is designed so that the strip-shaped transitional portion (drawing transitional portion) which is formed during the mask drawing process has a pitch having a relationship with that of the mask pattern at a particular cycle, the strip-shaped transitional portion on the mask may overlap the pattern formed on the FPD substrate. In such a case, a stripe pattern non-uniformity having a certain pitch may occur on the pattern on the FPD substrate. Such a pattern non-uniformity may be observed as a brightness non-uniformity on the display panel when a gray-scale image is displayed on the FPD in the form of a module.
Such a brightness non-uniformity is a phenomenon occurring due to a combination of two factors. The first factor is an optical diffraction due to a shift of about xc2x10.1 xcexcm in the formed pattern, which may occur when the dimensional and positional precision in the mask transitional portion is not so high (about xc2x10.1 xcexcm) and a remaining pattern on the substrate (e.g., the remaining pattern corresponds to a portion of the mask with a chrome film deposited thereon for a positive resist, and the remaining pattern corresponds to an opening of the mask for a negative resist) is provided on the transitional portion. The term xe2x80x9cremaining patternxe2x80x9d as used herein refers to a portion of the resist that remains after being cured. Such an optical diffraction may be observed as a lightness/darkness non-uniformity on the display panel, thereby resulting in a display non-uniformity.
The second factor is the pitch of the brightness non-uniformity. For example, the visibility of the above-described lightness/darkness non-uniformity (brightness non-uniformity) substantially varies depending upon the pitch of the non-uniformity. For example, when the lightness/darkness non-uniformity has a pitch of about 2 mm or more, the lightness/darkness non-uniformity is observed as vertical and lateral stripes on the display panel. However, when the pitch is about 1 mm or less, the lightness/darkness non-uniformity is generally not observed. One factor which increases the pitch of a lightness/darkness non-uniformity is a relatively long pitch of the pattern non-uniformity which is determined based on the least common multiple of the pitch of the mask drawing transitional portions and the dot pitch of the display device. When the ratio between the transitional portion pitch and the dot pitch is 56:41, for example, a drawing transitional portion overlaps a mask pattern for every 56 dots of pixels of the display device. In such a case, if the dot pitch is 307.5 xcexcm, for example, a lightness/darkness non-uniformity having a cycle of 17.22 mm occurs. The lightness/darkness non-uniformity having such a long cycle has a very high visibility, and may easily be observed as a display non-uniformity on the display panel.
Such a display non-uniformity due to a positional and dimensional error of a mask pattern may be avoided if the mask pattern precision can be improved. However, it has been confirmed that the deflection angle of an acousto-optical modulator may vary due to a change in the temperature, and it is well known in the art that the pattern precision may be affected by even a slight inclination of the rotational axis of the polygon mirror or a rotational non-uniformity of the polygon mirror. Therefore, it is difficult in practice to further improve the pattern precision. Other than those described above, there are still other factors which decrease the pattern precision. For example, although the scanning and stepping precision of the optical stage and the optical head is provided by using a laser length measuring devices, the positional precision of the laser length measuring device may not always be synchronized with the optical system for producing the drawing beam. It is believed that the positional precision of the laser length measuring device has at least some variation. Moreover, in order to improve the pattern precision, it is necessary to keep constant the stage stepping interval. Therefore, each element of the stage driving system, e.g., a ball screw or a linear motor, is required to have a very high precision.
As described above, improving the mask pattern precision itself is very important and effective in order to obtain a display device having a high quality, but also is very complicated and difficult. Improving the mask pattern precision may require, for example, a very costly correction mechanism provided in the drawing device. As a result, it becomes very time-consuming and costly to obtain a high-precision mask, which will be reflected upon the cost of a product which is produced by using the mask.
According to one aspect of this invention, there is provided a photomask for use in production of a display device, the photomask having on its surface a plurality of drawn areas and a transitional portion between two adjacent ones of the drawn areas. A pattern is formed on at least one of the drawn areas. A ratio between a pattern pitch and a transitional portion pitch is an integral ratio such that a length defined by a least common multiple of the pattern pitch and the transitional portion pitch is 1 mm or less.
In one embodiment of the invention, the ratio between the pattern pitch and the transitional portion pitch is 1:1.
In one embodiment of the invention, the ratio between the pattern pitch and the transitional portion pitch is 3:4, 2:3, 1:2, 1:3 or 1:4.
In one embodiment of the invention, the pattern and the transitional portion do not substantially overlap each other.
In one embodiment of the invention, the pattern pitch corresponds to a dot pitch of the display device.
In one embodiment of the invention, the transitional portion pitch is determined by controlling a scanning width of a head of a mask drawer for drawing the photomask and a pitch with which a stage of the mask drawer is moved.
In one embodiment of the invention, the pattern is a signal line.
In one embodiment of the invention, a pattern drawing operation is performed by a raster scan method.
According to another aspect of this invention, there is provided a method for producing a TFT substrate. The method includes the steps of: forming a gate bus line on a substrate; forming a semiconductor layer on the substrate; forming a source bus line so that the source bus line crosses the gate bus line and is electrically connected to the semiconductor layer; and forming a pixel electrode so that the pixel electrode is electrically connected to the semiconductor layer. At least one of the gate bus line, the source bus line, the semiconductor layer and the pixel electrode is formed by using a photomask. The photomask has on its surface a plurality of drawn areas and a transitional portion between two adjacent ones of the drawn areas. A pattern is formed on at least one of the drawn areas. A ratio between a pattern pitch and a transitional portion pitch is an integral ratio such that a length defined by a least common multiple of the pattern pitch and the transitional portion pitch is 1 mm or less.
According to still another aspect of this invention, there is provided a method for producing a display device. The method includes the steps of: forming a TFT including a gate bus line, a source bus line, a semiconductor layer and a pixel electrode on a first substrate; forming a counter electrode on a surface of a counter substrate which faces the first substrate; and providing a display medium layer between the first substrate and the counter substrate. The step of forming the TFT includes the step of forming at least one of the gate bus line, the source bus line, the semiconductor layer and the pixel electrode by using a photomask. The photomask has on its surface a plurality of drawn areas and a transitional portion between two adjacent ones of the drawn areas. A pattern is formed on at least one of the drawn areas. A ratio between a pattern pitch and a transitional portion pitch is an integral ratio such that a length defined by a least common multiple of the pattern pitch and the transitional portion pitch is 1 mm or less.
In one embodiment of the invention, the display device further includes a color filter, and the color filter is formed by using the photomask.
According to still another aspect of this invention, there is provided a method for producing a display device. The method includes the steps of: forming, on a first substrate, a plurality of first signal lines extending in a stripe pattern along a first direction; forming, on a second substrate which opposes the first substrate, a plurality of second signal lines extending in a stripe pattern along a direction different from the first direction; and providing a display medium layer between the first substrate and the second substrate. At least one of the step of forming the first signal lines and the step of forming the second signal lines uses a photomask. The photomask has on its surface a plurality of drawn areas and a transitional portion between two adjacent ones of the drawn areas. A pattern is formed on at least one of the drawn areas. A ratio between a pattern pitch and a transitional portion pitch is an integral ratio such that a length defined by a least common multiple of the pattern pitch and the transitional portion pitch is 1 mm or less.
The function of the present invention will now be described.
In the photomask of the present invention, the pattern pitch and the transitional portion pitch have a integral ratio such that the least common multiple thereof is 1 mm or less. Therefore, a mask pattern non-uniformity in a transitional portion, if any, always occurs with a cycle of 1 mm or less. When using such a photomask which may have a pattern non-uniformity with a relatively short pitch, the produced pattern (e.g., a gate line pattern, a source line pattern, or an insulator pattern) on the substrate may also have a pattern non-uniformity with a relatively short pitch. If a pattern is formed on a TFT substrate by using such a photomask and the TFT substrate is used in a display device, even when the pattern non-uniformity acts as a diffraction grating, there will be no brightness non-uniformity which can be observed on the display panel by an observer because the pitch of the pattern non-uniformity is short.
Moreover, when the ratio between the pattern pitch and the transitional portion pitch is set to 1:1, it is possible to suppress the occurrence of a periodic pattern non-uniformity even if there is a positional shift of the pattern in the transitional portion. A pattern formed on a substrate using such a photomask does not have a periodic pattern non-uniformity. Therefore, when such a substrate is used in a display device, it is possible to better suppress the occurrence of the brightness non-uniformity, or the like.
Furthermore, when the ratio between the pattern pitch and the transitional portion pitch is set to 3:4, 2:3, 1:2, 1:3 or 1:4, the transitional portion will have an increased pitch, whereby it is possible to increase the size of each area to be drawn. Therefore, it is possible to increase the drawing pitch by which the drawer draws a mask, whereby it is possible to reduce the mask production time.
By preventing a pattern and a transitional portion from substantially overlapping each other, it is possible to suppress the occurrence of the positional and dimensional shift of the pattern in the transitional portion.
Thus, the invention described herein makes possible the advantages of: (1) providing a photomask for use in the production of a large-screen display device with which it is possible to reduce the occurrence of the display non-uniformity in the produced display device without improving the positional and dimensional precision of the mask pattern; and (2) providing a method for producing a TFT substrate and a display device using such a photomask.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.